Highly energy efficient buildings are generally designed to avoid uncontrolled ingress or egress of air. As some air exchange is necessary to remove stale air and replace it with fresh air, it is desirable to first remove heat from the stale air to avoid losing the heat with the air. A heat recovery ventilator is used for this purpose.
A heat recovery ventilator includes a heat exchanger with two discrete air passageways, one for stale air exhaust and the other for fresh supply air. As the exhaust air passes out of the enclosure through the heat exchanger, it gives up its heat to the fresh supply air entering the enclosure through the heat exchanger. Accordingly the heat is “recovered” in the ventilator during the ventillation process and hence the name “heat recovery” ventilator.
A problem occurs with heat recovery ventilators in situations where the fresh supply air is at below freezing temperatures. As the stale air generally contains moisture, once it passes up heat, the moisture will freeze in the stale air exhaust passageway. Eventually ice build-up will block the passageway preventing the exhausting of stale air.
Different mechanisms have been proposed in order to defrost the ventilator, for example, as disclosed in Canadian Patent No. 2,059,195 and Canadian Patent No. 2,140,232. According to the latter, two actuators and respective valves or flaps are used to close the exhaust outlet and fresh supply air inlet. Stale air is thereby redirected to return back through the fresh supply air passageway to defrost the stale exhaust air passageway. This is carried out periodically, typically before the passageways totally freeze up. A drawback to this arrangement is the cost and complexity associated with utilizing two actuators each controlling separate valves or flaps.
The former patent suggests that instead of having two actuators it is possible to block the cold supply inlet and divert stale air exhaust back through the fresh supply air passages in the heat exchanger. While this does eliminate a valve or flap and an actuator it does present its own problems. As the actuator and flaps are disposed adjacent the cold supply there is a possibility of their freezing, thereby rendering them inoperable. Furthermore while the fresh air supply is closed stale air is recirculated. It would be preferable if the unit were capable of admitting at least some fresh supply air to commingle with the stale air.